Custom socially acceptable to a wise man (5)
I believe the answer is:
usage
'custom' is the definition.
(I know that custom can be written as usage)
'socially acceptable to a wise man' is the wordplay.
'socially acceptable' becomes 'U' (as in U and non-U language).
'to a wise man' becomes 'sage' (sage is a kind of wise man).
'u'+'sage'='USAGE'
(Other definitions for usage that I've seen before include "Custom - treatment" , "Interest of old" , "Treatment; tradition" , "Customary way in which language is spoken or written" , "Acceptable language" .)